In the field of surface science, intensive studies have been conducted on material surface properties such as adsorptivity and wettability. Surface modification techniques for forming organic monomolecular film, referred to as SAMs (Self-assembled monolayers) (see Whitesides, G. M. et al., Annu. Rev. Biomed. Eng., 3, 335-373, 2001 (hereinafter referred to as [Non-Patent Document 1])), on the surface of materials have been developed and a number of attempts have also been made to actively vary the properties of the material surface.
Recent years have seen the development of microfluidic devices that have integrated functions for separation, mixture, reaction, and detection of samples. One of crucial parameters for determining the performance of those devices that make use of electrokinetics is the zeta potential or the potential on the solid-liquid interface (see Kirby, B. J. et al., Electrophoresis, 25, 187-202, 2004 [hereinafter referred to as Non-Patent Document 2]). In the fields of electrochemistry and biochemistry, intensive studies have been made to apply patterned surface modifications to microfluidic devices to thereby control the wettability and material adsorptivity thereof.
Such studies on the material surface have led to measurement techniques for evaluating surface properties. The techniques which have been suggested include a technique for visualization of the structure of material surfaces using the electron microscope (see Lopez, G. P. et al., Langmuir, 9, 1513-1516, 1993 [hereinafter referred to as Non-Patent Document 3]), a technique for evaluation of surface modifications using the thickness of SAMs or the type of molecules contained therein (see Yang, x. M. et al., Appl. Phys. Lett., 69 (26):4020-4022, 1996 [hereinafter referred to as Non-Patent Document 4]), a technique for evaluation of surface properties using wettability (see Lopez, G. P. et al., Science, 260, 647-649, 1993 [hereinafter referred to as Non-Patent Document 5]), and the like.
For the performance evaluation and the optimum design of the microfluidic device mentioned above, it is effective to quantitatively measure the zeta potential on the flow path surface within the device. That is to say, the surface modification pattern applied onto the wall surface of flow paths in the device or the two-dimensional distribution of the zeta potential established by mixed liquid samples can be quantitatively visualized to thereby evaluate contributions to device performance.
Several methods have been devised for measuring the zeta potential in microfluidic fields. The methods include the streaming potential method (see Oldham, I. B. et al., J. Colloid Sci., 18, 328-336, 1963 [hereinafter referred to as Non-Patent Document 6]), the current monitoring method (see Sze, A. et al., J. Colloid Interface Sci., 261, 402-410, 2003 [hereinafter referred to as Non-Patent Document 7]), the method for measuring electroosmotic velocities to calculate the zeta potential using the Helmholtz-Smoluchowski equation (see Sinton, D. et al., J. Colloid Interface Sci., 254, 184-189, 2002 [hereinafter referred to as Non-Patent Document 8]), and the like. But these methods are all dedicated to the measurement of the average zeta potential across the flow path.
On the other hand, the nano-scale laser induced fluorescence imaging method developed by two of the inventors (see Kazoe, Y. and Sato, Y., Anal. Chem., 79, 6727-6733, 2007 [hereinafter referred to as Non-Patent Document 9]) was the first one which enables the measurement of a two-dimensional distribution of zeta potentials within the device. The nano-scale laser induced fluorescence imaging method employs the fluorescent dye (red) that is negatively ionized in an aqueous solution and the evanescent wave that occurs by total reflection of light on the interface between different refractive indices (see Axelrod, D. et al., Ann. Rev. Biophys. Bioeng., 13, 247-268, 1984 [hereinafter referred to as Non-Patent Document 10], and Japanese Patent Application Laid-Open No. 2007-85915 [hereinafter referred to as Patent Document 1]). The evanescent wave diminishes exponentially in intensity with increasing distance from the interface. It is thus possible to produce the wave on the wall surface of flow paths, thereby allowing an area within the distance of a few hundred nanometers from the wall to be irradiated therewith. With the fluorescent dye mixed in an aqueous solution flowing into the path, the concentration distribution of the negatively charged fluorescent dye in the vicinity of the wall surface varies depending on the wall zeta potential. Therefore, the fluorescence intensity upon excitation by the evanescent wave depends on the zeta potential. Accordingly, the distribution of the fluorescence intensity teaches the zeta potential distribution.
Note that other than the zeta potential, there are also disclosed a technique for obtaining the distribution of pH, in J. Coppeta et al., Experiments in Fluids 1-15, 1998 [hereinafter referred to as Non-Patent Document 11], and a technique for obtaining the distribution of temperatures, in Y. Sato et al., Meas. Sci. Technol. 14, 114-121, 2003 [hereinafter referred to as Non-Patent Document 12].
To obtain the two-dimensional distribution of zeta potentials according to the nano-scale laser induced fluorescence imaging method, the CCD camera is used to acquire fluorescence intensity images. However, since the acquired image may contain a fluorescence intensity distribution caused by an excitation light intensity distribution, a reference image has to be acquired to make compensation therefor. For this reason, it is difficult to visualize the two-dimensional distribution of zeta potentials from the image itself captured by the CCD camera, which is not suitable for real-time measurement. Furthermore, it is also necessary to make compensation again for different measurement positions or flow path shapes. Thus, this method has drawbacks such as measurement errors caused by shifts in position or the intricacy of the measurement technique.